X-ray scans and X-ray computed tomography (CT) scans are used in a wide range of medical and industrial settings to generate images of structures within a three-dimensional subject that are otherwise invisible to the naked eye. For example, CT scans of medical patients are used in a wide range of pathologies including, but not limited to, identification of tumors, infectious process, infarctions, calcification, hemorrhage, and trauma.
While X-ray and CT scans are widely used, the X-ray radiation that is generated during the scans may increase risk of cancer for the patient being scanned. One goal is to control the exposure to radiation for humans and other living subjects that undergo the CT scan while also generating images with sufficient clarity and resolution to be useful in diagnostic procedures. This balance between the need to use the lowest X-ray radiation while maintaining an image quality required for optimal identification of pathology is known as ALARA “as low as reasonably achievable.” During a CT scan, an X-ray source, such as an X-ray emitting tube, rotates around the longitudinal axis of the patient while emitting X-ray radiation. Some of the X-ray energy passes through the patient to a detector on the opposite side of the patient from the X-ray emitter, while the patient absorbs a portion of the X-ray energy. Certain structures in the patient, such as bones, fat, muscles, and water, absorb different amounts of energy resulting in different amounts of X-ray energy passing to the detectors. The information on the absorbed dose by the detectors at each position of the X-ray tube around the body is transferred to image processing systems that are known to the art that generate two and three dimensional models of structures in the patient using the individual X-ray images.
During a CT scan, a subject is typically positioned on a movable member such as a sliding table. The sliding table moves through an annular opening that is surrounded by the series of X-ray detectors. The X-ray source revolves around the patient as the patient moves through the annular opening on the sliding table to generate a series of X-ray images. The patient absorbs a portion of the X-rays, while other X-rays pass through the patient and reach the detectors during the imaging process. There is a tradeoff between the quality of images and the amount of X-ray energy that the patient absorbs. For example, a lower energy X-ray scan produces images that have high noise level and/or poor contrast, which can obscure details about structures within the patient. Increasing the intensity of the X-ray exposure improves the quality of images, but the patient also absorbs additional X-ray radiation during the scan.
The amount of X-ray radiation delivered to the patient and the quality of the image are dependent on the tube current (mAs) and the peak tube voltage (kVp). The power output of the tube is expressed as a product of the tube current and peak tube voltage. The dosage that the patient receives from the CT scanner corresponds to the portion of the emitted power from the tube that is absorbed by the tissue in the body of the patient multiplied by the length of time that the patient is exposed to the X-rays from the tube. For example, a higher level of tube current and higher peak tube voltage generally results in a higher quality image, but delivers X-ray radiation to the patient at a correspondingly higher rate. Existing CT systems generally have several options for constant tube voltage, which typically include 140 kVp, 120 kVp, 100 kVp and 80 kVp. In these systems, the tube voltage and current are selected based on the patient's weight, the type of scan, and the radiologist's preference of image quality.
These competing constraints present challenges for operating a CT system. The human body size ranges from a size of a baby to an obese adult. Different tube voltages and tube currents are needed to optimize the study for each body size. In addition, the image quality and tolerance to image noise are different for various body sizes. Many current systems use body weight to guide tube voltage and current settings. One drawback to selecting tube current and voltage based on the patient's weight is that weight alone is not necessarily a good representation of the quantity of X-ray energy a subject will absorb. For example, a subject who is at a particular weight, but is shorter and overweight, will typically absorb more X-ray energy than a subject who is at that same weight, but is taller and not overweight. One other challenge is that every group of radiologists may have different preferences of image quality based on experience, patient population and prevalence of specific pathologies.
Automating the current and voltage selection process reduces some of the variability in image quality between CT scans performed in patients with different body size and habitus. These methods are based on the water equivalent diameter (WED) of the scanned area of the patient's body as calculated from the scout view. Typically, one or few algorithms determine the change in the tube current based on the scanned WED and a preference of image quality. However, the preference of image quality is different for every body size. In a smaller body size, less image noise is typically preferred. Institutions typically have patients with differing demographics and many institutions specialize in particular types of services and diseases with a specific preference to CT scan image quality. A graph comparing the image quality preferences of two different institutions for different WEDs is shown in FIG. 4. Consequently, an automated procedure with a predetermined algorithm of change in tube current and voltage with change in scanned WED may be insufficient to meet the needs of a variety of institutions. The process of optimizing CT scans with the currently known processes may require many trial and error CT scans Improved automation of tube current and peak voltage selection that is based on the specific customer preferences of image quality is therefore desirable.